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In this amazing video of multiple camera angles of Endeavour landing and post-landing processing, at around the 15:45–15:56 minute mark, we see a large propeller running in front of the Orbiter.

It seems to be perfectly aligned with the Orbiter's centerline (see orange mark).

What is its purpose, and what is it called?

I first thought it might be there to provide airflow to carry fumes into a defined direction in case of a spill of Hydrazine or other nasty chemicals, but actually, there is a lot of people working downwind near the back of the Orbiter. Also, by this point, the guys in the hazmat suits seen earlier in the video, would have already removed those chemicals, I guess.

So, the only other reason I can come up with, is cooling. However, isn't it a little bit small and too far away for that, or am I misjudging the power of a ~4m propeller? And what exactly is it cooling? The tiles, the crew compartment, the crew van?

Large propeller fan in front of Space Shuttle Orbiter

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It turns out, my first hunch was half-right: the purpose of this large fan is not so much to carry away, but to disperse both toxic and explosive chemicals.

It is called the Vapor Dispersal Unit, and I vastly under-estimated its power:

The Vapor Dispersal Unit is a mobile wind-making machine able to produce a directed wind stream of up to 45 mph. It is an adaptation of a standard 14-ft. agricultural wind machine designed to protect fragile agricultural crops from frost damage or freezing. It is used by the recovery team to blow away toxic or explosive gases that may occur in or around the orbiter after landing. The fan can move 200,000 cubic feet of air a minute.

For the metrically challenged:

  • 45 mph = 72 km/h = 20 m/s
  • 14 ft = 4.3 m
  • 200000 ft³ = 5700 m³

Examples of the kinds of gases that are expected, are hydrogen, monomethyl hydrazine, and hydrazine and ammonia.

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    $\begingroup$ The linked document lists "hydrogen, monomethyl hydrazine, and hydrazine and ammonia". I added it to the answer. $\endgroup$
    – Jörg W Mittag
    Commented Dec 10, 2019 at 7:09
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    $\begingroup$ @Arvo That's not for the metrically challenged - NASA are the metrically challenged ones! $\endgroup$
    – J...
    Commented Dec 10, 2019 at 13:12
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    $\begingroup$ FYI, while the Orbiter was in the reentry phase during a nominal end of mission, there was at least one switch throw, performed by the pilot, that was done solely to make the ground crew's job safer. $\endgroup$
    – Digger
    Commented Dec 10, 2019 at 16:24
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    $\begingroup$ @OrganicMarble Prolly not one you saw too often (since you guys would never let us attain a "nominal end of mission" state)! After a (nominal) OMS reentry burn, the PLT set the 4 L,R OMS He PRESS/VAP ISOL switches to CL, thereby mitigating the (remote) chance that OMS oxidizer vapor could contact OMS fuel vapor (not a good thing for hypergolic propellant). $\endgroup$
    – Digger
    Commented Dec 18, 2019 at 0:28
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    $\begingroup$ @OrganicMarble Also, independent of c of g concerns, one could make a case that the forward RCS dump procedure, with the subsequent securing of the subject system, could prevent a bad day from getting worse if the Orbiter suffered a hard nose slapdown on landing (lots of relatively fragile plumbing located a good distance away from the main landing gear center of rotation). $\endgroup$
    – Digger
    Commented Dec 18, 2019 at 0:30

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